Electrode DC-offset rejection (AC coupling / DC servo)

Every electrode-electrolyte interface sits at a half-cell potential, and mismatches between the recording and reference electrodes produce a slowly-varying DC offset that can reach tens of millivolts — three to four orders of magnitude above the microvolt neural signal. Fed straight into a high-gain amplifier, that offset would saturate the output, so the front-end must reject it while preserving the signal.

The classic solution is AC coupling: a series input capacitor with a pseudo-resistor feedback path sets a sub-hertz high-pass corner that blocks DC but passes spikes and most LFP. The alternative, needed when true low-frequency or DC-coupled content matters, is a DC-servo loop — an integrator in feedback that senses and cancels the output offset while leaving the passband intact.

The engineering difficulty is the extremely low corner frequency required. A sub-hertz high-pass with realistic capacitors demands feedback resistances in the gigaohm-to-teraohm range, which are built as pseudo-resistors (subthreshold MOS elements) or duty-cycled resistors. These occupy area, drift with temperature and process, and can add noise and distortion, so offset rejection is a real front-end design axis rather than an afterthought.